Microtubules, along with actin and intermediate filaments, are the three components of the cellular cytoskeleton. Microtubules are hollow tube-like structures composed of linear protofilaments that are assembled from dimers of α- and β-tubulin. These rigid structures are larger than both actin and intermediate filaments, have an outer diameter of about 25 nm with a luminal diameter of about 18 nm (Howard (2001) Mechanics of Motor Proteins and the Cytoskeleton (Sinauer Associates, Inc.)), and can be between 1 μm and 1 mm long (Wade & Chretien (1993) J. Struct. Biol. 110:1-27). Microtubules are dynamic filaments, able to polymerize and depolymerize in a regulated manner, and are therefore used in many cellular processes (Desai & Mitchison (1997) Ann. Rev. Cell Dev. Biol. 13:83-117; Mitchison & Kirschner (1984) Nature 312:237-242). One necessary process is the formation of the mitotic spindle, where microtubules play a critical role in chromosome segregation. In addition, microtubules act as the protein tracks for molecular motor proteins, such as kinesin and dynein.
Tubulin exists as a stable heterodimer containing a non-exchangeable (α-subunit) and exchangeable (β-subunit) guanine nucleotide (GTP) binding site (Amos & Schlieper (2005) Curr. Opin. Struct. Biol. 10:236-241). In a growing microtubule, dimers add faster to the plus end. GTP in the β-subunit is slowly hydrolyzed to GDP, but the GTP in the α-subunit is sterically blocked from exchange by the β-subunit, and its GTP is never hydrolyzed (Downing & Nogales (1998) Curr. Opin. Cell Biol. 10:16-22; Lowe, et al. (2001) J. Mol. Biol. 313:1045-1057; Nogales, et al. (1999) Cell 96:79-88; Nogales, et al. (1995) Nature 375:424-427). If GTP-bound α/β dimers add to a growing microtubule faster than hydrolysis proceeds, it produces what is known as a GTP cap, which stabilizes the end of the microtubule. Subsequently, if hydrolysis catches up to the microtubule end, the microtubule becomes unstable and depolymerizes in what is called ‘catastrophe’. This rather unique behavior of microtubules is referred to as dynamic instability (Desai & Mitchison (1997) Ann. Rev. Cell Devel. Biol. 13:83-117; Mitchison & Kirschner (1984) Nature 312:237-242), and can be regulated by various microtubule associated proteins, as well as by regulation of tubulin dimer concentrations.
Each tubulin subunit is composed of three domains: an N-terminal nucleotide-binding domain, an intermediate domain, and a C-terminal domain, composed of helices α11/α12 and the acidic C-terminal “tails”, which together constitute the binding region for microtubule-based motor proteins (Lowe, et al. (2001) J. Mol. Biol. 313:1045-1057; Nogales, et al. (1995) Nature 375:424-427). The α/β tubulin heterodimers interact with each other longitudinally (head-to-tail) to form long, rod-like polymers called protofilaments. Between 10 and 15 (but generally 13 in vivo) parallel protofilaments interact laterally (side-by-side) to form a hollow cylindrical structure. The structural polarity of the microtubule gives rise to different rates of polymerization at either end. The rapidly polymerizing end of the microtubule is designated as the plus-end, while the slow growing end is called the minus-end. Different microtubule-based motor proteins have been shown to move along the microtubule track in one direction or the other, as well as regulating the polymerization and depolymerization dynamics.
A number of bacterial homologues of tubulin have been identified and structurally characterized. Among these are FtsZ (Lowe & Amos (1998) Nature 391:203-206), BtubA/B (Jenkins, et al. (2002) Proc. Natl. Acad. Sci. USA 99:17049-17054; Schlieper, et al. (2005) Proc. Natl. Acad. Sci. USA 102:9170-9175; Sontag, et al. (2005) J. Cell Biol. 169:233-238), TubZ (Chen & Erickson (2008) J. Biol. Chem. 283:8102-8109; Larsen, et al. (2007) Genes Dev. 21:1340-1352) and RepX (Pogliano (2008) Curr. Opin. Cell Biol. 20:19-27). While FtsZ is a very distant relative of eukaryotic α/β tubulin, BtubA/B are more closely related, and may even have been horizontally transferred into bacteria from eukaryotes (Schlieper, et al. (2005) supra). BtubA/B proteins form dimers in a manner very similar to α/β tubulin, and these dimers can form protofilaments in a GTP-dependent manner (Schlieper, et al. (2005) supra; Sontag, et al. (2005) supra). Although microtubule-like structures have not been observed for BtubA/B, the protofilaments are able to associate, forming twisted pairs as well as bundles. The overall protein fold of BtubA/B is very similar to eukaryotic tubulin (˜1.7 Å root mean square displacement between ˜360 amino acid α-carbons) despite modest sequence identity (31%-37%). In the crystal structure of the BtubA/B dimer, BtubA (corresponding to β-tubulin) was bound to GDP while the nucleotide binding site of BtubB (corresponding to α-tubulin) was empty.